Pressure-Sensitive Adhesive Art
Normally tacky pressure-sensitive adhesive (hereinafter referred to by the abbreviation "PSA") compositions suitable, for example, for use in adhesive tapes must have an art-recognized (1952 Fall Symposium, Division of Paint, Varnish and Plastics Chemistry, American Chemical Society) four-fold balance of adhesion, cohesion, stretchiness and elasticity. PSA coated tapes have been produced and sold for at least a half century.
The early PSA tapes relied upon natural rubber for the elastomeric base and wood rosins as tackifiers to provide adhesive compositions with the requisite four-fold balance of properties. While tackified natural rubber provided a PSA composition which was of commercial significance, improvements in such compositions were sought because of the expanded expectation level of performance of PSA compositions. Various improved PSA compositions were thus developed.
Ionic polymerization produced block copolymer elastomers such as linear AB and ABA block copolymers which were likely candidates for the elastomer base in the PSA compositions and many were incorporated into such compositions to produce adhesives having high performance characteristics. For example, Harlan (U.S. Pat. No. 3,239,478) produced PSA compositions based on ABA block copolymer, tackifier resin and extender oil, recognizing that improved tack and cohesive strength could be obtained despite a heavy loading of extender oil. Miller (U.S. Pat. No. 3,519,585) produced an improved PSA composition having high peel strength, creep resistance and tack by blending AB and ABA block copolymers with a tackifier resin.
Other elastomer candidates for preparing PSA compositions include radial teleblock copolymers and multiarm star block copolymers. The various polymer structures described by the terms "branched", "radial" and "star" are not the same. "Branched" is a generic term indicating a nonlinear structure which may contain various polymeric subunits appended to various places on a main polymer chain or backbone. Such structures are typically complex in nature and may be derived by free radical or cationic polymerization. The term "radial" generally refers to branched polymer structures obtained by linking individual polymeric segments to yield a mixture of polymers having four or fewer arms joined centrally. The term "star" describes the structure of a multiarm polymer with copolymer arms which are joined together at a nucleus formed of a linking group which is virtually a point relative to the overall size of the remainder of the polymer structure. Non-terminating coupling agents, those in which the polymerizing anionic structure is retained, are generally preferred as linking agents for "star"structures.
While several references disclose preparing adhesive compositions or PSA compositions employing radial teleblock copolymers and multiarm star block copolymers, none have recognized that novel anionically-prepared copolymers containing organometallic-substituted styrene may be used to prepare PSA compositions nor that such compositions exhibit unusual melt viscosity characteristics as well as excellent adhesive properties. For example, St. Clair (U.S. Pat. No. 4,444,953) describes asymmetric star block polymer prepared by terminally linking together a mixture of styrene-isoprene AB block polymers and isoprene homopolymers. The melt viscosity of such asymmetric star polymers is generally significantly higher than their linear counterpart. Marrs et al (U.S. Pat. No. 3,658,740) discloses the preparation of PSA compositions by combining branched block copolymers with linear block copolymers, tackifiers and organic solvents. Marrs, PSA formulation requires a solvent as a critical element to provide an adhesive formulation which bonds to a wide variety of substrates but fails to address the need for hot melt processability. Nash (U.S. Pat. No. 4,163,764) discloses the preparation of PSA compositions employing a two-step process in which a monovinyl-arene monomer, such as styrene, is first polymerized, followed by a second stage where diene monomer and additional initiator are added and the resulting polymerized product linked to give linear or radially-branched polymers. These polymers, when formulated with tackifiers, exhibited superior tack and creep resistance. Feeney et al (U.S. Pat. No. 4,288,567) employs a branched block copolymer described in Prudence (U.S. Pat. No. 3,949,020) and relies upon a solution preparation process to achieve an adhesive composition having increased tack, faster molten solution time, and improved tack retention in hot melt blends.